1. Field of the Invention
The present invention relates to the production of ultrapure water for use in the electronics industry, drug industry, etc. and particularly to a process and system for purifying pure water or ultrapure water by retreating pure water or ultrapure water to produce ultrapure water of extremely high purity.
2. Prior Art
In the electronics industry where ultrahigh density integrated circuits (ULSI circuits), represented by a 4 megabit dynamic memory, are produced, pure water of ultrahigh purity, i.e., ultrapure water, is required in large quantities for rinsing semiconductor products after each production step. For example, quality requirements for ultrapure water become increasingly severe as the integration of LSI circuit becomes higher, as shown in Table 1; and a reduction particularly in TOC (total organic carbon), living microbes and DO (dissolved oxygen) is a big task.
TABLE 1 ______________________________________ Integration Item 1M 4M 16M ______________________________________ Resistivity (M.OMEGA.-cm at 25.degree. C.) &gt;17.5 &gt;18.0 &gt;18.0 Fine particles (counts/ml) 0.1 .mu.m &lt;20 0.08 .mu.m &lt;10 0.05 .mu.m &lt;10 Living microbes (counts/100 ml) &lt;10 &lt;5 &lt;1 TOC (.mu.g C/l) &lt;50 &lt;30 &lt;10 Silica (.mu.m SiO.sub.2 /l) &lt;10 &lt;3 &lt;1 DO (.mu.g O/l) &lt;100 &lt;50 &lt;50 ______________________________________
In conventional production of ultrapure water, a starting water is treated by a pretreatment apparatus which is a combination of an coagulation and precipitation unit, a sand filtration tower or an active carbon tower, etc. The resulting pretreated water is treated by a primary pure water production apparatus which is a combination of an ion exchange resin tower, a decarbonation tower, a reverse osmosis membrane unit, a vacuum degassing tower, a mixed bed type cartridge demineralizer, etc. The resulting primary pure water is treated in a secondary pure water production apparatus (a subsystem) which is a combination of an ultraviolet sterilization unit, a mixed bed type cartridge demineralizer and an ultrafiltration membrane unit, to obtain ultrapure water. This ultrapure water is fed to various semiconductor production apparatuses.
FIG. 8 shows the conventional arrangement of a pretreatment apparatus, a primary pure water production apparatus, a subsystem and semiconductor production apparatuses.
That is, in FIG. 8, a starting water 1 is passed through a pretreatment apparatus 2 and a primary pure water production apparatus 3 to obtain a primary pure water. The pretreatment apparatus 2 and the primary pure water production apparatus 3 are ordinally installed in a building apart from a factory building in which semiconductor production apparatuses are installed. The primary pure water is introduced into a high purity water tank 4 installed in the factory building, via a primary pure water pipe 7.
The primary pure water stored in the high purity water tank 4 is treated by a subsystem 5 to be converted to an ultrapure water. This ultrapure water is introduced into the semiconductor production apparatuses 6 via an ultrapure water pipe 8. Any excess of ultrapure water is separated off just before the semiconductor production apparatuses 6 and is returned to the high purity tank 4 via a return pipe 9. Thus, the high purity water tank 4, the subsystem 5, the ultrapure water pipe 8 and the return pipe 9 form a loop, whereby ultrapure water is circulated constantly.
The ultrapure water used for a rinsing purpose at the semiconductor production apparatuses 6 becomes a waste water 11 discharged via a waste water pipe 10. The portion of the waste water 11 having a relatively good quality is recovered by a waste water recovery unit, etc. (not shown in FIG. 8) and, in some cases, is utilized as a starting water 1.
It is known that in the arrangement illustrated by FIG. 8, the ultrapure water obtained from the subsystem 5 is significantly reduced in its purity when it stops flowing and is stagnant. The reason is that the TOC, inorganic salts, etc. contained in the pipe, etc. dissolve in the ultrapure water at the portion of the pipe, etc. contacting the ultrapure water. Therefore, the ultrapure water has heretofore been circulated constantly in the loop formed by the subsystem 5, the ultrapure water pipe 8, the return pipe 9 and the high purity water tank 4, to prevent the deterioration of the purity of the ultrapure water.
However, the subsystem 5 and the semiconductor production apparatuses 6 are installed relatively apart from each other in many cases, and the length of the ultrapure water pipe 8 reaches 100-500 m in some cases. When ultrapure water is passed through such a long pipe, the purity of the ultrapure water tends to deteriorate even though the water is circulated constantly without being allowed to stagnate. In view of the trend toward higher density integration of LSI circuits and the fact that the quality requirements for ultrapure water are corresponding becoming increasingly higher, as shown in Table 1, it is important to introduce ultrapure water obtained from the subsystem 5, into the semiconductor production apparatuses 6 without any substantial reduction in quality.
As described above, with the conventional ultrapure water production system, it was very difficult to introduce the ultrapure water obtained therefrom, into the semiconductor production apparatuses without a reduction in the quality of the ultrapure water. Further, with the adoption of more complex semiconductor production steps and more diversified semiconductor production apparatuses, the length of the ultrapure water pipe 8 has become larger necessarily, which has resulted in a reduction in ultrapure water quality.